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Thread: Sizing boiler allowing for future expansion

  1. #16
    Retired Defense Industry Engineer jadnashua's Avatar
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    Unless the boiler was running full blast on the coldest day and never shutting down to keep the house warm, it is oversized! Dana has explained it numerous times, and you can read them if you search...a good way to see how much heat you actually used is to take your fuel use and match it up with the degree-day history files for your zipcode. You should have access to your fuel bills, and you can download the degree-day data for the period. That will tell you how much energy you actually put into the house for that period. You'd want to do that for a full heating season, and if you have the data, you could do it for several to get an average, since not all winters are the same. A little margin wouldn't hurt, but you don't want much. If you use huge setbacks overnight, the extra margin will help recover faster, but once you achieve the setpoint, that extra margin costs money in both efficiency and life of the unit.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  2. #17
    In the trades Dana's Avatar
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    Quote Originally Posted by DPJ View Post
    "Maybe they were unduly influenced by the size of the old boiler and the 180' of baseboard."

    OK, so I asked why the need for a 125k+ boiler, and he said 600 btu/ft x 180 gives the jumping-off point (108,000) He also said it will provide for quicker recovery for the new indirect. I have currently been running a tankles coil in the old boiler.
    Downright LOUSY reasons all around!

    Size the indirect for the hot water heating load (sizing it for largest tub-fill is about right), and run it as a "priority zone". With 74K of boiler output you can run a 2gpm shower FOREVER, (that's more than 2x the burner output of a typical 40 gallon gas-fired standalone tank!) but if you have a standard-sized tub that you want to fill at 8-10gpm you'll want at least 30 gallons of tank,i f kept at 140F. You can go somewhat smaller if using high storage temps. (Using a 26 gallon ErgoMax and "sharing" output with radiation you'd want to run it higher than 140F during the heating season.)

    The length of the baseboard determines the water temp required to deliver the load at the design temperature, which is going to be well-under 180F, even with the updated 134' fin-tube length. The fact that you've been running a tankless coil, at your annual fuel use it means that your space heating load at the 99% design temp of about +5F is well under 70K, and could easily be under 60K. So it's likely that the highest temp you'd ever need on the baseboard is about 155F, and if you insulate & air seal you can probably bring it down to 145F at the outside design condition.

    With outdoor reset control on the boiler the average will be much lower. By running the baseboard at lower temp means the distribution & standby losses to the semi-conditioned basement will be lower, and the system-efficiency much higher.

    On an old-school boiler you'd have to add in some near-boiler plumbing to protect the boiler (and the flue) from acidic condensation, but in the MPO-IQ84 that protection is internal to the boiler, no extra system design necessary on that front.

    DO run the FSA calculator using your fuel use and the fact that it's an embedded coil (==massive shoulder & summer season standby losses) and let the tool auto-adjust your heat load estimate. (When I run it using your boiler size using Hartford with an older design temp of -1F, even plugging in 1300 gallons/year it's coming up with heat loads under 50K, which is still credible.).
    Last edited by Dana; 09-17-2012 at 08:14 AM.

  3. #18
    In the trades Dana's Avatar
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    Taking issue (if only slightly) with Jim's comments :

    " On the coldest design day, the boiler should be running constantly..."

    and

    " Unless the boiler was running full blast on the coldest day and never shutting down to keep the house warm, it is oversized"

    With a non-modulating boiler sized EXACTLY to the heat load at design temp it would have burns that may go for over an hour or two on the coldest MORNING of the year (while still keeping you warm), but as the day grows warmer it'll still cycle off for extended periods, since the heat load falls below the boiler's output with rising outdoor temps, lowered even further by solar gains. If it runs constantly

    With a modulating boiler it should be running pretty much constantly for the entire month of January if sized exactly to the heat load, since the heat load even at the daily highs will usually be within the modulating range of a modulating-condensing boiler. But with fixed output boiler (like 99% of all oil-fired boilers) it'll be cycling, even on the coldest day of the year, but the burns will be several 10s of minutes long on those days in a decently designed system if sized perfectly.

    In practice you can only go so low for burner output with oil as a fuel, and even the smallest burners are usually 2x oversized for new code-min houses, if only 1.5x oversized for a leaky antique house.

    You're existing boiler is probably getting at-best 55% efficiency according to the FSA calculator, but an MPO-IQ84 would be hitting in the high-80s. Plugging in 1100 gallons/year and a Hartford climate, with an 129KBTU "old boiler w/ internal coil" it's estimating a heat load under 40K but an efficiency just under 55%. Swapping in the smaller high-tech boiler + indirect would cut your fuel use by a bit more than 1/3. Using the bigger -IQ115 with 98K of output would be a mistake, it would cost a bit more up front and every year there-after.

  4. #19

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    Keep in mind also that if you have mostly or all fin tube radiators that those are not the best case scenario when switching to a modcon. They're low mass and will not create a situation where you'll have the delta-t that you're looking for. Just another reason to size the unit for exactly what you need, no more.

    I had a Burnham modcon installed two years ago. It's GROSSLY over sized for my house as it stands now. Luckily my unit can be dialed back so that I'm only using a portion of its heating capacity. My indirect tank is 50 gal and even for that priority I have the boiler dialed back. I've never run out of hot water.

    The idea is to create a situation where the output water temp doesn't match the returning water temp. One that happens the water temp will just rise to the maximum limit set on the curve and the boiler will shut off until the water reaches the low end temp.

  5. #20
    In the trades Dana's Avatar
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    The mass of any oil-fired boiler is many times that of a gas-fired mod-con, and those with smart-controls will automatically exercise that mass with bigger hysteresis to keep burn lengths reasonable. But even the smallest & smartest oil boiler would have a problem if the planned addition is heated with low-mass radiation unless mass is added to the system, which is what prompts pondering the Ergomax configuration.

    As currently zoned the current fin-tube lengths are manageable for the MPO-IQ084, if the boiler's controls fully utilize the available hysteresis range (which is considerable compared to dumb-controlled oil boiler that aren't inherently protected from cool return water), but with a low outdoor reset temp on the MPO-IQ115 the burn lengths will be shorter, average efficiency lower. The smallest zone has 62' of fin-tube into which the boiler is dumping 85,000BTU.

    If the outdoor reset is trying to adjust it to 120F AWT (about as low as you'd want to go with fin-tube- output is less predictable below that), the fin-tube is only emitting ~200-220BTU/hr/ft, so x 62' it's ~13,000BTU/hr into the zone, with 72,000BTU/hr (1200BTU/min) of excess. With something on the order 100lbs of water-equivalent mass in the boiler + zone the water temp rises (1200/100)= 120F/minute- the burns will be SCREAMING short unless the water temps were allowed a great deal of hysteresis. Since the fin-tube puts out more heat at the higher temps you'd get at least 3 minutes of burn out of it, but probably not 5.

    With the -IQ84's 74K of output you'd have ~60,000BTU/hr of excess (1000BTU/min) and the temps would rise at 100F/min, and the temps would still be pretty short. With an Ergomax (or any 25-30 gallon insulated tank) buffering the system it effectively triples the thermal mass, and triples the burn time at any temperature or hysteresis.

    At 140F AWT you get ~330BTU/ft or (x 62'=) ~20,000BTU/hr of output from the fin-tube, which would still leave 65,000BTU of excess with the -IQ115, (almost 1100BTU/min) but only 55,000BTU/hr of excess (920BTU/min) out of the -IQ84 for a rise-rate of 92F/min un-buffered, 31F/min if buffered with 25-30 gallons.

    At 180F AWT only half the output of the -IQ084 would be emitted into the zone with a T-rise rate of ~ 37F/min (unbuffered), and it would be well under half with the -IQ115, with a T-rise rate of ~80F/min (unbuffered), more than 2x the rate of the smaller boiler.

    These are just a few points on the curve to get a feel for it rather than doing the formal integral calculus to come up with more precise burn times, but know that if the outdoor reset doesn't allow a very substantial hysteresis you're looking at very short burn lengths, in either case, but they'll be much shorter and lossier with the -IQ115 than with the -IQ084. Adding thermal mass would be of benefit with either version, but it would become essential with the bigger boiler, no matter how much hysteresis is programmed in, since the temp is still climbing fast at 180F. Getting 10+ minute minimum burn times would be ideal- even 5 minutes would be OK, but that would not be in the cards with the bigger boiler unless mass is added.

  6. #21
    DIY Junior Member SD4US's Avatar
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    I'm trying to follow Dana's very nicely detailed (many thanks for it) heat load calculation in post #5 (using my own numbers) but there is a step there, where something seems wrong:

    "so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers"

    Either you factored in something unknown into the above equation or it could be a simple mistake, as 60 heating degrees x 780 BTU per degree-hour = 46,800 BTU/hr, which is no longer very close to DPJ's online heat load figure (75,000).

    You felt (I guess correctly) that 78,000 BTU/hr was a significant over-estimate and further lowered it based on DHW and old boiler efficiency and came up with an end result of 55,000 BTU/hr (which you felt was still on the high side). If the quoted calculation was indeed incorrect, that final number would be only 33,000 BTU/hr.

    Does this make any sense?

  7. #22
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Quote Originally Posted by DPJ View Post
    Based on some online heat loss calculators
    Better than nothing but nearly useless in renovation work, as the variables are many and the options with such software few. Let's start with a proper heat load. You have enough baseboard to support the larger boiler but that is only a one factor. The indirect water heater should not be added in the the heat load and a 40 is probably the same cost as 35-see MegaStor or Trin&Stor indirect water heaters. Foamed your rim joist yet?

  8. #23
    In the trades Dana's Avatar
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    Quote Originally Posted by SD4US View Post
    I'm trying to follow Dana's very nicely detailed (many thanks for it) heat load calculation in post #5 (using my own numbers) but there is a step there, where something seems wrong:

    "so with a maximum of 780 BTU/degree- hour, that means your heat load at + 5F is less than or equal to 60 x 780= 78,000BTU/hr as an absolute max, which is pretty close to your online heat load numbers"

    Either you factored in something unknown into the above equation or it could be a simple mistake, as 60 heating degrees x 780 BTU per degree-hour = 46,800 BTU/hr, which is no longer very close to DPJ's online heat load figure (75,000).

    You felt (I guess correctly) that 78,000 BTU/hr was a significant over-estimate and further lowered it based on DHW and old boiler efficiency and came up with an end result of 55,000 BTU/hr (which you felt was still on the high side). If the quoted calculation was indeed incorrect, that final number would be only 33,000 BTU/hr.

    Does this make any sense?

    Yup- it was indeed a typo/math-error (I need an editor! :-) ), and...

    yes 46.8K is a realistic combined-use number. With the oversizing factor contributing to lower as-used efficiency of 75%, not 85% you're then looking at (46.8K x 75/85)- 41.3K for the combined use. If hot water was 20% of it, then the heat load portion is actually only 33K.

    But running the numbers on the actual fuel use against the actual heating degree days is important. If the oil supplier stamps a "K-factor" on the billing, it's the same information expressed differently: HDD/gallon rather than BTUs (or gallons)/HDD, but the conversion is 5th grade arithmetic (which I apparently need to re-take! :-) )

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